1. Field of the Invention
The invention relates to estimating a mobile unit's location using time difference of arrival (TDOA) of communication signals. In particular, the invention relates to adjusting the estimated location of the mobile unit in a cellular communication system using TDOA.
2. Description of the Related Art
Recently there has been a great deal of interest in determining the location of mobile devices. One area that is of particular interest is the ability to determine the location of cellular phone users in certain circumstances. For example, the U.S. Federal Communication Commissions (FCC) has mandated that the location of a cellular user be determined when the user dials 911. In addition to the FCC mandate, it is envisioned that other applications that can take advantage of knowing a user's location will be developed.
Various techniques have been used to determine the location of a mobile unit. For example, the Global Positioning System (GPS) is a satellite system that provides users equipped with a GPS receiver the ability to determine their location anywhere in the world. While GPS provides world wide coverage, it suffers from several drawbacks. One such drawback to the GPS system is that in general a user must have a clear view of four GPS satellites to accurately determine their location. For a cellular user this can be a significant hindrance to the use of GPS because many cellular users are in urban areas where objects, such as tall buildings, may shadow the user so that they are unable to see the required number of satellites. The problem is further exacerbated when the cellular user is indoors in a building.
Other techniques have been developed that attempt to exploit some of the characteristics of the cellular system itself to aid in the determination of the location of a mobile unit. One such technique is the Time Difference of Arrival (TDOA) technique. The TDOA technique can be used when the actual transmission times of received signals is known, or when the transmission of the received signal occurs at a known periodic rate. For example, if a mobile unit receives signals that are transmitted from at least two base stations, and the signals are transmitted by the base stations at the same time, or the signals are synchronized to each other, the mobile unit will be able to determine the relative difference in time that the two signals are received. The differences in time that the two signals are received corresponds to the difference the distance traveled by each signal from the respective base station to the mobile unit.
The TDOA between two signals establishes a set of points that maintain the constant time difference, and corresponding travel distance, between the received signals. The set of points defines a hyperbolic surface representing possible locations of the mobile unit. By making multiple, simultaneous, TDOA measurements a family of surfaces can be generated with the intersection of these surfaces providing an estimate of the location of the mobile unit.
A problem can arise when using the TDOA technique if the signal received by the mobile unit has been delayed. For example, in a typical wireless communication system, a signal emitted from a base station reflects off surfaces, creating multiple instances of the signal that may travel several distinct paths as they propagate through the wireless channel between the base station and a mobile unit. This phenomenon is commonly referred to as multipath. Each of the multipaths traveled by the signal instances is typically a different distance than the other paths, resulting in the multipath signals being received at different times from each other, as well as being delayed from the time that a direct line-of-sight signal would arrive at the mobile unit. The direct line-of-sight signal represents the actual distance from the base station to the mobile unit.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of the mobile unit and structures that create the multipath. Thus, the amount of delay of the received signal is also time varying.
The multipath characteristics of a channel can also affect the signal received by the mobile unit in other ways, resulting in, among other things, attenuation of the signal in addition to excess delay of the signal. Attenuation in the signal strength results from, among other things, energy from the signal being absorbed as the signal propagates through the medium and is reflected off objects. In addition, the signal received at a mobile unit is attenuated due to geometric spreading of the signal as it propagates through the wireless channel.
Excess delay is the difference between the time it takes the signal to travel a multipath route from the base station to the mobile unit and the time it would have taken if the signal had traveled a direct line-of-sight path between the base station and the mobile unit. For example, there may be no direct line-of-sight path between the base station and the user because, for example, the user is shadowed from the base station by a building. In this situation the signal received at the mobile unit will travel a distance greater that the actual line-of-sight distance between the base station and the mobile unit because the signal will have to be reflected off objects to “get around” the obstruction and reach the mobile unit. The increase in distance traveled by the signal introduces additional, or excess, delay into the time of arrival of the signal, resulting in an error in the TDOA measurement, increasing the inaccuracy of the estimated location of the mobile unit.
Excess delay can also be introduced even if there is a direct line-of-sight path between the base station and the mobile unit. For example, the signal received from the direct line-of-sight path may be attenuated such that it is not sufficiently strong to permit the mobile unit to make a timing measurement. Therefore, one of the multipath instances of the signal will be the first signal received by the mobile unit with sufficient strength to allow a timing measurement to be made.
Due to these and other problems, signals in a typical communication system, particularly ones operating in a multipath environment, experience excess delay, thereby increasing the inaccuracy in location estimates made using the TDOA technique. Therefore, there is a need to improve the reliability of TDOA measurements and the associated location estimate.